This invention relates to a suspension system for a vehicle and more particularly to a suspension system which comprises a shock absorber of a damping force characteristic variable type provided between a sprung member and an unsprung member.
In general, the vehicle suspension system comprises a shock absorber provided between a sprung member (the body side) and a unsprung member (the wheel side) for damping the vertical oscillation of a wheel. There are various types of shock absorbers. By way of example, the shock absorbers of a damping force characteristic variable type are grouped into one which can change a damping force characteristic (characteristic of a different damping coefficient) into two steps (higher level and lower level) and another which can change the damping force characteristic into many steps or steplessly.
The shock absorber of a damping force characteristic variable type is basically controlled by setting the damping force of the shock absorber to the lower level side (i.e., SOFT, side) when the damping force of the shock absorber acts in the oscillation-stimulating direction with respect to the vertical oscillation of the body and setting the damping force of the shock absorber to the higher level side (i.e., HARD side) when the damping force acts in the oscillation-restraining direction so that oscillation-restraining energy is made larger than the oscillation-stimulating energy transmitted to the sprung member. Thus, comfortableness to ride in and running stability are improved.
There have been proposed various methods for judging whether the damping force of the shock absorber acts in the oscillation-stimulating direction or the oscillation-restraining direction. For examle, Japanese Patent Application Laid-Open Gazette No. 60-248419 discloses a method for judging the direction of the damping force in such a way: It is judged whether the sign of the relative displacement between the sprung member and the unsprung member is the same as that of a differentiated value thereof, which is the rlative speed between the sprung member and the unsprung member. If the signs are the same, it is judged that the damping force acts in the oscillation-stimulating direction. If the signs are not the same, it is judged that the damping force acts in the oscillation-restraining direction. Also, Japanese Patent Application Laid-Open Gazette No. 61-163011 discloses a method for judging the direction of the damping force in such a way: It is judged whether the sign of the absolute speed of the sprung member is the same as that of the relative speed between the sprung member and the unsprung member. If the signs are the same, it is judged that the damping force acts in the oscillation-retraining direction. If the sings are not the same, it is judged that the damping force acts in the oscillation-stimulating direction.
However, in the vehicle having the shock absorber of a damping force characteristic multistep variable type, when the damping force characteristic is changed and controlled to generate an approximately ideal damping force, loud noise and oscillation are caused by the large change of the damping force characteristic of the shock absorber.
To solve the above-mentioned problem, the present applicant has proposed that the number of the damping force characteristic to be selected from three or more characteristics of the shock absorber are limited to two or so according to the running state (a vehicle speed, a lateral acceleration or the like), and that the damping force characteristic of the shock absorber is selected and changed on the basis of a given control rule only within the limited damping force characteristics (U.S. Pat. application No. 07/697,883). In this case, at the time of turning in which the lateral acceleration of the vehicle is increased, the damping force characteristic of the shock absorber is uniformly changed into the higher level side. At the time of constant turning in which body behavior is stable, it is desired that the damping force characteristic of the shock absorber is changed into the lower level side so as to seriously consider and realize the comfortableness to ride in, as compared with the time of transient turning in which the lateral acceleration is uniform.